The current system operates 20 zones with solenoid operated diaphragm valves. These 20 valves are 24Vdc 0.3 Amp.
do you know the model number / link to the valve? The first thing is to make sure these are indeed DC valves. Also, whether it’s latching or non-latching. If you can tell me more about the specifications of the valves, I can give you better advice.